Turbine engines, such as turbo prop or turbojet engines, include high speed blade rotor assemblies which operate inside of the engine casing. The blades on the rotor assemblies provide the propulsion force for the engine, and in order to operate properly and at peak efficiency, the blade tips must be properly spaced apart from tip sealing components coupled to the engine casing. The sealing components comprise a plurality of seal segments which are mounted on the casing radially of the blade tips. During engine operation the blades will expand and contract due to varying operating temperatures, rotational velocities, and the like. This, in concert with thermal and structural distortions of the engine case, results in variations in the spacing between the blade tips and the static seal segments, and can even result in contact between the blades and the seal segments. In theory, the space between the blade tips and the seal segments should be kept as small as possible, but the blade tips should never touch the seal segments as this would cause permanent wear damage to the seal material.
Efforts have been made to actively control the spacing between rotating engine blade tips and static seal segments so as to try to maintain the target minimal spacing while avoiding blade-seal contact. These active controls are designed to impart radial movement to the seal segments so as to attempt to maintain the target gap by moving the seal segments outwardly when the tip clearance decreases, and inwardly when the clearance increases. One such active control which utilizes heat to move the seals is disclosed in U.S. Pat. No. 4,928,240 granted May 22, 1990 to S. H. Davison, et al. U.S. Pat. No. 4,247,247 granted Jan. 27, 1981 to G. W. Thebert discloses a flexible seal shroud in a turbine engine which can be deflected toward turbine blade tips by means of pressurized gas introduced into a chamber adjacent to the seal shroud.
Solutions to the problem which involve selectively moving the blades or blade tips toward and away from the seal are also disclosed in the prior art. Examples of such solutions are found in U.S. Pat. No. 5,203,673 granted Apr. 20, 1993 to D. H. Evans; and in U.S. Pat. No. 5,263,816 granted Nov. 23, 1993. These patents disclose mechanical and magnetic active controls respectively.
Japanese Kokai Patent Application No. HEI (1989)-267,301, published Oct. 25, 1989 discloses an active control system for a turbine engine which may utilize heat to selectively position seal shroud segments relative to the turbine blade tips; or in a second embodiment, may use piezoelectric actuators to position the shroud segments relative to the blade tips. The piezoelectric control system includes a gap sensor which senses the actual gap between the sensor and the rotating blade tips, and stacks of piezoelectric elements connecting each shroud segment with the fixed engine casing. Selectively variable voltage is applied to the piezoelectric element stacks by a servoamplifier in order to increase or decrease the thickness of each piezoelectric element in the stack thereby controlling the thickness of the stack so as to move each shroud segment relative to the blade tips. The use of a stack of radially aligned piezoelectric film layers to actively control the gap between the blades and the seal shroud of a turbine engine such as is disclosed in the aforesaid Japanese Kokai is impractical due to the limited amount of movement that can be obtained thereby. Each film layer, at maximum voltage, could only increase its thickness by approximately one micron (10.sup.-6 meter). Thus if each stack consisted of as many as fifty film layers, the total possible range of motion would still be less than 0.005 inch. This limited range of motion is not acceptable in large gas turbine engines such as are found on commercial and military aircraft. Such engines require that a minimum of 0.050 inch range of active control motion be provided by the control system. The piezoelectric actuator disclosed in the Japanese Kokai is an impractical solution to the problem, however, some form of piezoelectric gap control actuator would be desirable since the ability to control distortion of a piezoelectric body with varying voltage provides very accurate and reliable control parameters.